1. Field of the Invention
The present invention generally relates to a control system for an automatic transmission. More specifically, the present invention relates to an automatic transmission control system for preventing disengagement malfunction of a frictional engaging element.
2. Background Information
In a geared automatic transmission in which each of gear ratios is achieved by selectively engaging and releasing a plurality of clutches and brakes (each generically referred to as a “frictional engaging element”) a disengagement malfunction of one (first) frictional engaging element (for example, it is adhered) may cause a secondary malfunction of a disengagement malfunction of another (second) frictional engaging element. Hereinafter, the phrase “disengagement malfunction” of a frictional engaging element refers to a malfunction in which a frictional engaging element remains engaged irrespective of a release command, encompassing not only a state in which it remains fully engaged (no relative rotation) but also a state in which it remains partially engaged to allow transmission of a portion of power.
In particular, when a certain one clutch or brake is involved in a disengagement malfunction, pressing down the accelerator pedal with a parking range (hereinafter, P range) or a neutral range (hereinafter, N range) being selected will cause the engine rotational speed to increase to a high engine rotational speed range because the engine is running with no load. In this situation, the disengagement malfunction of the first frictional engaging element may result in a case in which another (second) frictional engaging element (clutch or brake) is forced to rotate at a high speed. This high rotational speed poses a problem in that the second frictional engaging element (clutch or brake), which was operating normally, will become stuck resulting in a secondary disengagement malfunction.
In the case of automatic transmissions, which include a planetary gearing, such as the ones described in Japanese Laid-Open Patent Publication Nos. 2003-254429 and JP2005-163916A, when a clutch (e.g., a first frictional engaging element) is malfunctioning and can not be disengaged, if the accelerator pedal is depressed with a P range or a N range being selected, then the engine rotational speed will increase to a high engine rotational speed range as mentioned above. In particular, in these conventional automatic transmissions, with rotation of a ring gear that is an output rotary member held at zero, a sun gear and a rotary member that is fixed to the sun gear will rotate excessively, resulting in a brake (e.g., a second frictional engaging element), which is fixed to the above-mentioned rotary member, to become stuck due to frictional heat, and thus, malfunctioning (i.e., remaining engaged or partially engaged).
Therefore, in these conventional automatic transmissions, one of the frictional engaging elements is produced with a torque transmission capacity higher than required for normal torque transmission in order to avoid occurrence of disengagement malfunction in this the frictional engaging element. In other words, the frictional engaging element is produced with additional capacity to prevent the disengagement malfunction due to another frictional engaging element having a disengagement malfunction.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved automatic transmission control system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.